Formerly, standard methods for densifying refractory oxide coatings by converting chromic acid in the coating to chromium (III) oxide involved application of an oxidizing solution to a refractory coating, subsequent heating to approximately 1100.degree. F., followed by cooling. This operation had to be repeated many times in order to convert a sufficient amount of chromic acid to chromium (III) oxide to achieve a densified refractory oxide coating. One disadvantage of these methods was that temperature-sensitive substrates, such as aluminum substrates, could be deformed by heating to 1100.degree. F. Another disadvantage of these methods was that ten to fourteen application cycles were required before satisfactory densification was achieved, thereby increasing processing costs.
Densification methods were improved by Church (e.g., U.S. Pat. Nos. 4,007,020 and 4,077,808). The Church methods improved the densification process by requiring heating at temperatures of only 600.degree. F. However, even 600.degree. F. will soften temperature-sensitive substrates, such as aluminum. Further, many densification steps were still required.
The present invention marks an improvement over prior compositions and methods for densifying a refractory oxide coating. The present invention requires fewer processing steps, and lower temperatures (about 450.degree. F.) that reduce softening (referring to the annealing process of metals) and deformation of temperature-sensitive substrates.